Slope Failure Hazard in Canada's Atlantic Provinces: a Review

Slope failure hazard in Canada’s Atlantic Provinces: a review

Ian Spooner1*, Martin Batterson2, Norm Catto3, David Liverman2, Bruce E. Broster4, Kim Kearns5, Fenton Isenor6, and G. Wayne McAskill7

1. Department of Earth and Environmental Science, Acadia University, Wolfville, Nova Scotia B0P 1X0, Canada 2. Geological Survey of Newfoundland and Labrador, Department of Natural Resources, Government of Newfoundland and Labrador, PO Box 8700, St. John’s, Newfoundland and Labrador A1B 4J6, Canada 3. Department of Geography, Memorial University of Newfoundland, St. John’s, Newfoundland and Labrador A1B 3X9, Canada 4. Department of Earth Sciences, University of New Brunswick, Fredericton, New Brunswick E3B 5A3, Canada 5. ESRI, 380 New York St., Redlands, California 92373-8188, USA 6. Cape Breton University, P.O. Box 5300, 1250 Grand Lake Road, Sydney, Nova Scotia B1P 6L2, Canada 7. Nova Scotia Department of Transportation and Public Works, 107 Guysborough Road, Fall River, Nova Scotia B2T 1J6, Canada

*Corresponding author

Date received 26 February 2012 ¶ Date accepted 30 November 2012

Abstract Slope failures present a hazard around the world, with considerable impact on communication/ transportation corridors, resource sectors, and coastal and urban development. Although poorly documented in much of Atlantic Canada, mass movements are known to have resulted in 71 fatalities in Newfoundland. In the Ferryland, Newfoundland, disaster of around 1823, 42 fishermen were reportedly killed when a cave roof collapsed onto them. Debris floods and flows are widespread in areas of higher relief in both Newfoundland and Cape Breton Island, and tend to be most hazardous for highway engineering and community development. Rotational slumps in glaciomarine clays are evident along the major river valleys of Labrador and are an important consideration during hydroelectric development. In other areas of Atlantic Canada, earthflows affect transportation routes and involve movement of saturated sediment during spring thaw. Along the Gulf of St. Lawrence, Northumberland Strait and the Bay of Fundy coastlines, friable rocks, glaciotectonic activity, frost wedging, and coastal erosion have resulted in many small failures and incremental retreat of coastal cliffs. Ongoing climate change will have an impact on slope failure activity. Increasing summer frequency and intensity of thunderstorms and hurricane events, increased winter precipitation in some locations, and possible erratic freeze-thaw events would result in an increase in debris flows triggered by precipitation, and rockfalls triggered by freeze-thaw. Human utilization of coastal areas is also contributing to the frequency and hazard of slope failures across Atlantic Canada.

RÉSUMÉ Les ruptures de versant représentent un danger partout dans le monde et ont une incidence considérable sur les corridors de communication ou de transport, les secteurs des ressources et l’aménagement côtier et urbain. Bien qu’ils soient mal documentés dans une bonne partie du Canada atlantique, ces mouvements de masse sont réputés pour avoir causé 71 décès à Terre-Neuve. Dans la catastrophe de Ferryland, à Terre-Neuve,

vers 1823, 42 pêcheurs auraient été tués lorsque le plafond d’une caverne s’est effondré sur eux. Les inondations et coulées de débris sont courantes dans les zones de relief élevé à Terre-Neuve et à l’île du Cap-Breton, et elles tendent à être très dangereuses pour la conception des routes et le développement des localités. Des glissements rotationnels dans les argiles glaciomarins sont manifestes le long des vallées fluviales importantes du Labrador et sont un facteur important à considérer lors d’aménagements hydroélectriques. Dans d’autres régions du Canada atlantique, les coulées de terre perturbent les routes de transport et entraînent le mouvement de sédiments saturés durant le dégel printanier. Le long des côtes du golfe du Saint-Laurent, du détroit de Northumberland et de la baie de Fundy, des roches friables, de l’activité glaciotectonique, de la gélifraction et de l’érosion côtière ont donné lieu à de nombreuses petites ruptures et au recul graduel des falaises côtières. Les changements climatiques en cours auront une incidence sur les ruptures de versant. La fréquence et l’intensité accrues des orages et des ouragans pendant l’été, l’augmentation des précipitations hivernales dans certains endroits et la possibilité de gels-dégels erratiques pourraient se traduire par une augmentation des coulées de débris déclenchées par les précipitations et des éboulements déclenchés par les gels-dégels. L’utilisation humaine des zones côtières contribue également à la fréquence et à la dangerosité des ruptures de versant dans l’ensemble du Canada atlantique.

[Traduit par la redaction]

INTRODUCTION SLOPE FAILURE IN NEWFOUNDLAND AND LABRADOR Some authors consider landslides not to be a major hazard in Atlantic Canada (Cruden et al. 1989; Evans 2001). A variety of slope failure types has occurred in In a compilation of Canadian landslide disasters, Evans Newfoundland and Labrador, including sackung; rockfalls; (2001), who defined a disaster as “an incident with three or debris torrents and floods; channelized debris flows and more fatalities”, showed that only one such event out of 45 debris avalanches; rotational slumps; and gelifluction occurred in the region. However, it is misleading to assume creep. Many incidents have been documented, with over that landslides are insignificant in the area and inappropriate 80 having been recorded as impacting communities and not to recognize potential risks to life and property (Fig. 1). infrastructure (Batterson et al. 2006). Some have been fatal, Liverman et al. (2001) and Batterson et al. (2006) reviewed including two landslides in St. John’s (1936 and 1948) and geological hazards in Newfoundland and tabulated known the 1973 Harbour Breton landslide that killed four children. landslides there. Additional research, documented here, Landslides involving rotational slumping of large masses of indicates that 29 to 71 fatalities in Newfoundland alone sediment have been recorded on the west coast of the island, may have occurred from landslides, as well as 68 from including events at Port au Port East in 1994; near Journois, snow avalanches (Liverman et al. 2001, 2003; Batterson St. George’s Bay in 1996; at Daniel’s Harbour in 2007–2008; et al. 2006). In Nova Scotia, five fatalities associated with and at Trout River in 2009 (Fig. 1). The last two examples snow avalanches have been documented (Isenor et al. 2005; caused no loss of life, but resulted in considerable property Spooner et al. 2010). damage and the relocation of several residences. Other slope Our objective is to summarize the variety of slope failures have occurred in remote areas, where they have had failure processes that are active in Atlantic Canada, with little impact on society, but may need to be considered if examples of recent and/or significant events. We also provide development takes place in the future. an overview of slope failure research in the region and indicate where further study is warranted. We demonstrate that slope failures are common and have a significant Sackungen impact on communication/transportation corridors, the resource sector, coastal and urban development, and other A “sackung” (plural sackungen) is a deep-rooted, activities. We use the terms slope failure and mass wasting generally slow-moving slope failure involving large volumes interchangeably to describe the downslope movement of of bedrock. A sackung is identified on aerial photographs rock debris and soil in response to gravitational stresses. We by a series of linear scarps that face up-slope; these are use the term landslide when the responsible mechanisms called “anti-slope scarps”. Grant (1974, 1987) identified 24 and the velocity of movement allow the slope failure to examples, mostly developed in ultramafic rocks in western be classified in detail. This review is restricted to onshore Newfoundland. The slowness of motion, combined with impacts, although the offshore is equally prone to slope the absence of residential or commercial development in movements. We review slope failures province-by-province, areas where these occur, limit their significance as a hazard. emphasizing the characteristic slope failure styles for each In times of exceptional rainfall, movement may accelerate (Fig. 1). We do not include snow avalanche hazards in this (Evans and Clague 2003), and a rapid collapse into water review. might generate displacement waves.

Figure 1. Location of selected slope failure sites discussed in detail in the text. CBI = Cape Breton Island, NB = New Brunswick, NL = Newfoundland and Labrador, NS = Nova Scotia, PEI = Prince Edward Island, P.P. = Provincial Park.

The largest example of a sackung in Newfoundland is Bay. Although the failure was initiated post-glacially, at Norris Point in Bonne Bay (Figs. 1, 2), where a rock mass a more precise age of failure has not been determined. estimated at 10 km3 has moved vertically by approximately Visual examination from 1974 to 2003 suggested that no 100 m (Fig. 2; Grant 1974). The head of the failure consists discernable movement was occurring during that time. of a complex arrangement of scarps and fissures that The causal mechanism for the Bonne Bay and other align roughly parallel with regional jointing. Grant (1974) sackung throughout western Newfoundland remains suggested that the failure of these dense ultramafic rocks uncertain. The locations of the sackungen do not coincide (which were thrust over relatively weak shale and carbonate with fault escarpments, making tectonic disturbance during the early Paleozoic) was caused in part by the over- unlikely. As the majority of sackungen are located adjacent to steepening of the fjord wall during the last glaciation. glacially over-steepened valleys, the failures appear to have The deep-seated failure occurred along the basal thrust developed in response to the removal of laterally supporting contact and spreading resulted on all slopes along Bonne bedrock during glaciation, triggering lateral expansion

Figure 2. Sackung at Norris Point, Bonne Bay, Newfoundland. This is the largest of 24 examples of sackung in western Newfoundland identified by Grant (1974) and has an estimated volume of 10 km3. The arrow points to the headscarp.

when the ice ablated. Similar slope failures, albeit on smaller northeastern Avalon Peninsula, for example, outcrops scales, are associated with valley down-cutting and “valley are characterized by dipping, fractured and well-bedded rebound” in other areas of the province (Babcock 1977). latest Precambrian clastic strata. Excavations in these Slow moving slope failure has occurred in the Advocate strata necessitate construction of retaining walls and/or Mine, Baie Verte (Fig. 1). This large open-pit asbestos mine, stabilization of the fractured areas. Fractured slopes are developed in ultramafic rocks, is now abandoned; but in subject to periodic failure by block fall, topple, and grain 1996, a large rotational slump developed in the pit wall. flow of previously disaggregated bedrock. Most failures are Since then it has been monitored in case it might affect the spatially restricted and initiated by frost wedging. Bedding- adjacent highway. The removal of lateral support during plane slides are rare. During the springs of 1995, 1997, 2000, mining operations suggests a similarity with the Norris Point 2001 and 2002, several small rockfall events, each involving sackung and other examples in western Newfoundland. a maximum block size of about 2 m3 of rock, occurred along Pitts Memorial Drive in St. John’s, obstructing traffic to a minor degree. The dip of the strata is opposed to the angle of Rockfall the excavated slope, limiting the size of frost-wedged blocks. The potential for a major slope failure involving motorist Rockfall is a common process in Newfoundland and casualties or reconstruction of the roadway is low, although Labrador, due to the combination of relief, steep slopes a motorist was slightly injured after her car struck boulders resulting from glaciation, repetitive freeze-thaw conditions, on the road in the 2001 incident (Batterson et al. 2006). and jointed, faulted and stratified rock-units with multiple Rockfalls have often affected property and dwellings planes of weakness. Most incidents involve single blocks, below steep slopes, but surprisingly no structure-related and those involving large volumes of debris are rare. The deaths have been recorded. However, fatalities have occurred most serious reported incident, described below, relates to away from dwellings. The most severe injury in a residential the unusual cave roof collapse at Ferryland (Figs. 1, 3, 4). setting occurred in Portugal Cove (Fig. 1), northwest of Small-scale rockfall events have been widely reported and, St. John’s, where a woman lost a leg when a boulder struck although these rarely result in fatalities, they frequently her house. Fatal accidents have involved people at work or cause damage and traffic delays, and are costly to clean up during recreational activities. A number of these accidents and remediate. involved sea-cliffs and caves, a reflection of the working Numerous road cuts throughout Newfoundland history of the province (Liverman et al. 2001). have required stabilization to prevent rockfall. In the

Figure 3. Ferryland Harbour, eastern Newfoundland. The village of Ferryland is to the left (A). The arrow points to “Deadman’s Gulch”, believed to be the site of a rockfall that killed 42 fishermen.

Rockfall at Ferryland They arrived at the Gulch (as gulch it was then) and discovered that during the night its roof had fallen An intriguing story of what may be Canada’s fourth- in, burying the 42 fishermen — not one having been worst landslide disaster was recounted in the Newfoundland rescued. None of the bodies were ever picked up; having Quarterly of March 1902 (White 1902). White reported a been buried, no doubt, beneath the crushing mass of cave-in at Deadman’s Gulch in Ferryland Harbour that stone which had fallen on them. apparently occurred in 1823 (Figs. 1, 3, 4). Although the report is detailed, White did not indicate the source of his If this story is accurate, this is the worst geologically information. Corroboration of the story has proved difficult related disaster in the Province of Newfoundland and — few newspapers were published at the time, and archives Labrador, and the fourth worst landslide disaster in of those that were published have much of 1823 missing. Canadian history (Evans 2001), with more people being White describes “Deadman’s Gulch”, apparent on killed only in the Frank slide (75 fatalities in 1903), the Jane modern aerial photographs, as a gulch or notch in the Creek disaster at Britannia Mine (54 fatalities in 1915) and cliff edge, extending several hundred metres inland from the 1889 slide in Québec City (50 killed). the coast on the south side of the Narrows that lead into Ferryland Harbour (Figs. 3, 4). White describes the incident as follows: Rotational slumps

In 1823 or thereabouts it did not thus appear. It was Rotational slumps and earth flows also occur in then a mammoth cave, with an entrance to the ocean, Newfoundland and Labrador, notably along the banks of in and out from which fishermen were accustomed the Churchill River in central Labrador (Fig. 1), where they to go from time to time to secure bait fish that used have not had a significant impact on human activity, but to frequent the gulch. But in the year of which I write are a major consideration for future development. Thick forty-two fishermen in about 14–15 boats entered the sequences of glaciomarine mud overlain by sands and gravel cave to avoid being swamped by a severe storm of wind are found in the lower Churchill River valley, exposed in and rain, accompanied by the usual heavy sea which river cut-banks up to 40 m high. These banks are unstable, had overtaken them while on the fishing ground. with widespread rotational slumps occurring within the glaciomarine mud. Similar stratigraphy and settings are Residents of Ferryland heard a sound resembling a very found in other major valleys on the Labrador Coast, and loud clap of thunder, and the next morning: similar slope failures are visible on aerial photographs. Large landslides have also occurred in areas of glaciomarine clay on the west coast of Newfoundland, notably at Romaines (Forbes et al. 1995) and Journois (Fig. 1).

Churchill River. Several of these, including the largest example described above, have occurred during the past 30 years, whereas other features are much older. Debris flows are common in areas disturbed by sliding. Mudflows are observed at river level at the toes of larger slide features. Deformation structures beneath flow deposits indicate that such flows were rapid, shearing and loading underlying material. The glaciomarine sediments of the lower Churchill form a significant geotechnical problem in construction, and further research is required to better understand their susceptibility to liquefaction and slope failure.

Figure 4. Deadman’s Gulch at the entrance to Ferryland Harbour. Before the collapse, the site was believed to have been a sea cave that was frequented to catch baitfish and as a refuge during storms. The large boulder in the background is approximately 10 m across.

Figure 5. A large rotational slump on the east side of the Slope failure on the Lower Churchill River lower Churchill River, Labrador. The active sediment is glaciomarine mud and overlying sand and gravel. Sliding The lower Churchill River between Gull Island and was likely initiated by fluvial undercutting that over- Muskrat Falls shows numerous slope failures typical of steepened the slope. Scale bar is approximately 20 m high. many of the major valleys in coastal Labrador whose floors are underlain by glaciomarine silt and clay (Figs. 1, 5). As this area is the site of a proposed major hydro-electric dam, Debris flows the potential for slope failures needs to be evaluated. Three types of mass movement are identified in this Debris flows are the most common destructive form area. The exposed glaciomarine sediments show relict of landslide in Newfoundland and Labrador. Landslide deformation structures likely related to fluvial downcutting scars from debris flows are widespread and visible on of exposed sediments shortly after a drop in sea-level that aerial photographs. Debris flows are prone to occur where exposed the sediments. Active features include debris slides a combination of steep slopes and thick surficial sediment and debris-flow deposits. Sliding activity is initiated by cover is present. In general, a relationship exists between fluvial undercutting, which over-steepens slopes formed their occurrence and exceptional precipitation events. mostly of glaciomarine silt and clay. Subsequent saturation Although glaciomarine mud is found in coastal areas of likely plays a role in triggering movement. Sliding is both western Newfoundland, many of the documented examples translational and rotational and can involve large volumes involve till or re-mobilized colluvium. Debris flows have of sediment. The largest feature in the region has a surface resulted in several fatal incidents, including those at Harbour area of approximately 3 km2 and involves about 60 million 3 Breton and on South Side Road in St. John’s (Fig. 1). m of sediment. Sixteen other similar failures with surface On 1 August 1973, following several weeks of heavy areas of greater than 1 km2 and involving volumes greater 3 rainfall, a debris torrent occurred at about 3 a.m. in a gully than 20 million m are present in this stretch of the lower above the community of Harbour Breton (Fig. 1). The

landslide is thought to have involved two events, separated by about 30 seconds (Nolan et al. 1973). The first flow originated from near the top of a 170 m high slope and is considered to have triggered the second major event, which lasted about three minutes. About 1500–2000 m3 of material were involved in the landslide (Nolan et al. 1973). The material was initially deposited on the road (at an elevation of about 12 m above sea level), which served as a temporary catch basin, but then overflowed and continued downslope to the shore, engulfing four houses in its path; four people in the houses were killed. There were previous reports of a landslide at the same site about 20 years earlier, and in 1971 a smaller debris flow had deposited enough sediment to block the road. On 19 April 1994, a series of debris flow deposits blocked Riverside Drive in Corner Brook (Fig. 1). The flows originated on a north-facing slope in which bedrock Figure 6. Debris flow deposit at Trout River, Newfoundland. is overlain by a compact glacial diamicton and fluvial The landslide displaced silt and clay and occurred sand and gravel. The cause of the debris flows was likely following a period of heavy rain. Arrow points to house at related to inadequate drainage from the adjacent newly right in Figure 7. constructed Trans-Canada Highway. During construction of the highway, a major drainage ditch was cut. Following a period of intense snowmelt, water normally carried by this ditch was re-directed through the subsurface to the adjacent slopes above Riverside Drive, triggering the debris flows. No injuries resulted, although Riverside Drive was closed until mid-summer. A series of landslides occurred in the community of Trout River at the western edge of Gros Morne National Park between 6 and 13 June, 2009 (Figs. 1, 6, 7). The landslides displaced about 200 m3 of sediment, mostly silt and clay. The initial landslide occurred following a period of heavy rain (40 mm over 24 hours at Deer Lake, the nearest observing station). The second landslide occurred in the same area, with another part of the slope detaching, liquefying and flowing downslope (Fig. 6). Most of the material appears to have come from the vicinity of the headwall, flowing over the lower slope, which remained largely intact under the mud. Figure 7. Debris flow deposit at Trout River, Newfoundland. The debris-flow deposit impacted a residence at the base of The landslide, though small in volume, detached a house the slope, detaching it from its foundation and pushing it from its foundation and pushed it onto the adjacent road. onto the adjacent road (Fig. 7). Three other residences at the base of slope were evacuated because they were assessed by provincial authorities to be at risk from further landslides. significant loss of property, including several residences, the Residents of all four properties have subsequently been subsequent removal of others from the site, and the eventual relocated. relocation of 23 families. The April 2007 landslide was preceded by a smaller event that affected the same area on 20 October 2006, which covered an area of approximately 1000 m2. The 2007 Daniel’s Harbour landslide produced a Landslides at Daniel’s Harbour large debris fan that extended about 60 m in front of the slide area (Fig. 8). On 15 April 2007, a landslide occurred in the community The landslides at Daniel’s Harbour are the most recent of Daniel’s Harbour on the Great Northern Peninsula; of a series of similar events around that community. Aerial it covered an area of at least 5300 m2 and involved about photographs show two large landslide scars, one north and 110 000 m3 of material (Figs. 1, 8). The landslide resulted in one south of the recent slide. The southern slide covers

planes, resulting in collapsed blocks. Following the initial failure, rapid release of pressure on the sediment produced subsequent vertical stress fractures that also failed as the clay-rich layer continued to be squeezed coastward. Small fracture planes oriented parallel to the surface were noted on eroded blocks on the debris-fan surface, confirming that pressure release was a significant process acting on the till. The initial trigger for the landslide is uncertain. The landslide was not associated with a heavy rainfall event or unusually rough seas. This is confirmed by the presence of snow at the base of the cliffs along much of the coastline in the Daniel’s Harbour area at the time of the landslide, which indicates that storm waves had not earlier impacted the coastline. A major storm affected the west coast of Figure 8. Landslide at Daniel’s Harbour, Newfoundland. Newfoundland on 4 February 2007, when wave heights of The slide was characterized as a combination of 8 m were reported in Daniel’s Harbour area. Perhaps wave translational-slip failure and rotational-slip failure and impact at the base of the cliff from this event was the trigger occurred as a result of the saturation and subsequent loss by removing material from the base of the cliff; but the long of strength of a clay-rich layer underlying till. interval of time between the storm and the landslide makes this scenario less likely. Other smaller slope failures along the coast north of Daniel’s Harbour indicate that waves had approximately 1.28 hectares, whereas the slide to the north undercut the coastline in places. Furthermore, the 2006 covers about 6.24 hectares. Both scars (now fully vegetated) landslide at Daniel’s Harbour was not associated with a are evident on the earliest aerial photographs of the area significant rainfall event (though 7.5 mm of precipitation (1947) and neither is remembered by community residents. was recorded on October 16), or with high waves (based on It cannot be determined if the scars represent a single local wind speeds). landslide event or a series of landslides. Other considerably smaller landslides and landslide scars were noted north of Daniel’s Harbour. Since 2007, a small landslide occurred on SLOPE FAILURE IN NOVA SCOTIA 18 June 2008 approximately 230 m south of the 2007 slide area. This later slide was a rotational slip type failure with a vertical scarp of 8 to 10 m along an estimated length of Nova Scotia also suffers from frequent landslides, 10 to 20 m. An estimated 1500 to 2000 m3 of material was particularly in the northwestern highlands of Cape Breton dislodged during this slide. Island (Fig. 1; Spooner et al. 2010). Grant (1974, 1987, 1994) The sediments exposed in the landslide area at Daniel’s documented numerous incidents of rockfalls and topples Harbour are glacial and post-glacial. The lowest observed from the cliffs that define much of the shoreline of the unit is a poorly exposed silty clay bed approximately 1 m Cape Breton Highlands. Grant (1994) also noted extensive thick, overlain by a very compact glaciomarine diamicton movement on the deep glacially and fluvially cut gorges with a silt-clay matrix and containing pebbles to boulders found on plateaus. These gorges are bordered by steep rock up to about 50 cm in diameter. The compact nature of the cliffs from which rockfalls are a frequent event as witnessed sediment is likely a combined result of its compression by extensive talus slopes; some large-scale rock slumps beneath ice and the presence of a high proportion of (sackung) have also been recognized (Grant 1994). A large carbonate material, which acted as cement on the fine- landslide appears to have taken place approximately 100 grained component of the sediment. The diamicton is years ago along the Aspy Scarp, and another near the mouth overlain by 1–2 m of loose nearshore/beach sand and gravel of the Cheticamp River (Fig.1; Grant 1994). (Proudfoot and St. Croix 1991), which were deposited The largest recent landslide consisted of a series of slope during an interval of raised sea level following glacial retreat. failures that caused near complete destruction of the road at The 2007 landslide was a combination of a translational Kellys Mountain on Cape Breton Island in 1982 (Figs. 1, 9). slip failure and a rotational slip failure, and likely occurred as A creosote wood culvert had been installed earlier to take the result of saturation and subsequent coastward movement runoff from the south side of the highway to a gully on the of the clay-rich layer that underlies the compact till observed north slope, guiding the flow into St. Anns Harbour. The in the coastal exposures. As the clay-rich layer became principal cause of the 1982 slide was likely water seepage saturated, it lost much of its cohesive strength. The pressure from the deteriorated culvert. This, combined with water from the overlying till caused it to fracture along vertical from spring thaw and rainfall, saturated the soil surrounding the culvert and caused erosion at the base of the slope, which

resulted in a number of failures in late spring 1982. Bedrock underlying the gully may also have created natural failure planes for the saturated soil. The landslide was characterized as a translational slip failure that developed into a debris flow.

Figure 10. A large rotational slump in Middleton, Nova Scotia. This landslide was associated with fluvial erosion of 20 m-high slope along the Annapolis River when the water table was elevated activating a failure plane between impermeable clay-rich till and overlying outwash sediment.

frequent, these landslides were not considered a significant hazard (Grant, 1994). Grant (1994) postulated that these landslides occurred as a result of mid- to late-spring snow avalanching or snowmelt, or possibly during rain-on-snow Figure 9. Debris-flow deposit at Kelly’s Mountain, Cape events when the ground was saturated and partly snow-free. Breton Island, Nova Scotia. This landslide has been Based on his observations, Grant (1994) determined that characterized as a translational failure that developed into “hydrology is clearly the operative process” in the landslide a debris flow. activity. Wahl (2003) determined that two stratigraphic relationships are linked to most of the failures: the presence Other smaller landslides frequently affect transportation of highly compact, impermeable clay-rich lodgement till routes in Nova Scotia, most commonly in spring, as thawing overlain by highly permeable colluvium; and impermeable of frozen ground leaves slopes saturated. A large rotational weathered bedrock in the form of saprolite overlain by slump occurred on the north side of Highway 201 near permeable colluvium. Both contact boundaries act as a slip Middleton in late April 2003 and resulted in prolonged surface, when groundwater accumulates at the colluvium/till highway closure (Figs. 1, 10). Site investigation indicated or bedrock interface within the activation zone, producing that the Annapolis River was actively eroding the toe of a an effective glide plane for initial translational movement. 20 m-high slope, which exhibited irregular topography and Air-photograph interpretation indicates that an increase displayed evidence of previous movement, including tension in landslide activity in Cape Breton Island has occurred cracks. The stratigraphy of the site is typical of the region: during a 30-year period beginning in the 1960s. Examination greater than 4 m of fluvial/glaciofluvial sand and gravel of monthly precipitation trends led Finck (1993) to propose overlies sandy-clayey till and, in some cases, glaciomarine that significant increases in the precipitation levels during clay. Geotechnical analysis of the site indicates that two the month of April from 1980 to 1990 compared to the triggers most likely contributed to the slide: removal of toe interval from 1961 to 1979 may have contributed to failure. support and a rise in the water table activating the failure Finck (1993), Wahl (2003), and Wahl et al. (2003) noted plane between the outwash sediment and the impermeable that root decay and loss of soil strength associated with tree- clay-rich sediments. root rot from spruce budworm infestation may also have Finck (1993), Grant (1994), and Wahl (2003) have been of importance. They also commented on the strong identified numerous debris flow/avalanche scars in the Cape spatial correlation between dead softwood stands and the Breton Highlands (Figs. 1, 11, 12). These complex failures occurrence of slope failure. involve rock topple, rotational slip, translational sliding and Preliminary archival research on the history of landslides flow (Finck 1993, Wahl 2003, Wahl et al. 2003). Although in Nova Scotia has shown frequent minor incidents (Spooner et al. 2010). A preliminary review of articles from the

SLOPE FAILURE IN NEW BRUNSWICK AND PRINCE EDWARD ISLAND

The combination of bedrock structure, glaciotectonic activity, frost wedging, and coastal erosion has resulted in the development of numerous small-scale landslide features in Prince Edward Island and New Brunswick, along with incremental retreat of coastal cliffs. As in other areas of Atlantic Canada, rockfalls, earthflows and small debris flows are common along highway cuts and steep river valleys. Larger slope failures have occurred in Quaternary glacigenic sediment at Lorneville Cove, near Saint John (Ruitenberg and McCutcheon 1978); in Quaternary marine sediment, diamicton, and the underlying Taylors Island Formation (Carboniferous; Barr et al. 2012); the Triassic Quaco Formation (Tanoli and Pickerill 1988) at Red Head and vicinity; and in Quaternary diamicton and the underlying Carboniferous Balls Lake Formation at Mispec Point and western Mispec Bay (Fig. 1). The landslide at Lorneville mainly involved slumping of layered marine clay, silt and sand along a well-defined curved plane of sliding in contact with dipping, underlying bedrock over a distance of about 300 m (Ruitenberg and McCutcheon 1978). This slope failure may have been coincidental with Figure 11. Debris-flow scar in the Cape Breton Highlands, a small regional earthquake (Broster and Burke 2011), but Nova Scotia. These failures are typically complex and may Wright (in Ruitenburg and McCutcheon 1978) reported involve rock topple, rotational slip, translational sliding that similar slides were previously known to have occurred and flow. in the Lancaster area, now a western extension of Saint John. Overloading of post-glacial marine terraces and coastal bluffs due to suburban development and saturation of the deposits has further enhanced the susceptibility of the area to slope failure, particularly in Saint John. Elsewhere in the Saint John area and St. Croix Lowlands, Precambrian rocks are subject to frost wedging, and weathered surfaces undergo limited frost creep, although no significant failures are evident. Failure of rocks along the Bay of Fundy involves exploitation of joints by frost action, in conjunction with undercutting by rising sea level and tidal action. This generally takes the form of rockfalls, block failure and toppling, although small rotational events locally develop along curved joint surfaces. The interior of Prince Edward Island and the late Paleozoic Figure 12. Debris-flow scars in the Cape Breton Highlands, lowlands of New Brunswick are not subject to significant Nova Scotia. Failure has been associated with excess water slope failures due to their shallow topographic relief; for at the contact between colluvium and underlying weather this reason, no notable events have been recorded in the bedrock or till. Fredericton and Moncton areas (Broster 1998). Exceptions in these areas occur where man-made excavations produce Cape Breton Post from 1900 to 1925 indicates that railway steep cuts that expose joints and bedding planes that dip out transportation was frequently disrupted by minor landslides of the exposure. Such occurrences are most dangerous when and washouts, usually following heavy rain. clay-filled steeply dipping joints are exposed due to removal of confining rock, as in open-pit mining (Park and Broster 1996). Sugar Loaf Mountain near Campbellton has been the

site of frequent small rock slides, but most slides occurring with marine undercutting induced by rising sea level, which in remote areas of high relief in central New Brunswick go is about 3 mm per year throughout the region (Shaw et unreported. In general, slope failures in New Brunswick al. 1999). Locally, saturation of the bedrock and overlying and Prince Edward Island have not posed significant threat Quaternary deposits resulting from agricultural practices to human life, largely because slope movements have been (irrigation, disruption of drainage) has contributed to debris relatively small in urban centres and larger movements have flow and creep failures. occurred in uninhabited areas. Erosion is generally marked by incremental block failure, making generalizations from calculations of short- term rates suspect. Rates for deep-seated bedrock failures Lorneville Cove landslide have exceeded 80 cm/a at several locations, including Cape Kildare (Fig. 1, Genest and Joseph 1989), Cape Gage, and Between 8:00 a.m. on 26 November 1977 and 2 a.m. on Panmure Island (Fig. 1). On Prince Edward Island, erosion of the following day, a landslide occurred along the southwest bedrock-supported cliffs at rates of about 10–50 cm/a is also shore of Lorneville Cove, on Saint John Harbour about 12 evident at East Point (John Shaw, personal communication), km west of the centre of Saint John. The slide developed in High Bank, Point Prim, Fort Amherst National Historic Site, layered marine clay and silt, with sand interbeds overlying MacCallums Point (Catto 1998), Cap Egmont (Fig. 1), Cabot patches of diamicton and bedrock. The failure involved the Beach Provincial Park (Fig. 1, Catto 1998), and Cape Tryon. slumping of a large wedge of the clay, silt and sand, which Monitoring of coastal cliffs is relatively limited, consisting overlay bedrock. Ruitenberg and McCutcheon (1978) largely of spot checks. The potential scale and extent of reported that the head scarp along the main failure plane coastal erosion of bedrock shores in Prince Edward Island, demonstrated a vertical offset of about 4 m, likely resulting as well as along the Acadian Peninsula of New Brunswick, from rotation of the major slump block and movement of warrants attention (Fig. 1). the slide toe into the water. The failure plane as recognized Joint systems in bedrock on Prince Edward Island by surface tension cracks that defined a circular head scarp resulted from a combination of lateral expansion due over a distance of almost 300 m, extended approximately to removal of sediment by coastal erosion and vertical 100 m back from the edge of the cove at its farthest point. expansion due to glacioisotatic rebound. The rate of The curved surface of sliding along the lower failure plane exploitation of these joints by mass wasting is controlled was estimated to be up to 10 m thick and in contact with primarily by their orientation with respect to prevailing bedrock along more than 80 m of sliding surface (Ruitenberg winds and the frequency of freeze-thaw cycles, but also by et al. 1976). The bedrock surface beneath the slide slopes the amount of coastal undercutting of weaker shale and seaward; throughout the area, this surface is streamlined, fine sandstone units. Freeze-thaw exploitation is dependent glacially polished, and intensely fractured. upon microclimate, which controls the number of cycles, The Lorneville Cove landslide occurred after a period of and therefore the rates can vary substantially on opposite unusually high rainfall, and it is likely that the zone around sides of an individual cove or peninsula. Another factor is the failure plane was saturated by groundwater. A seismic the fracture pattern within the rock unit, which facilitates tremor was reported to have been felt by a local resident water percolation below the surface. The tensile strengths of (Ruitenberg and McCutcheon 1978), but the archived data all the rock units in Prince Edward Island are significantly for the Dominion Observatory do not record an earthquake less (~30%) than the stress induced by freezing. Where at the time (Broster and Burke 2011). The tremor was likely fracture planes are narrow or tortuous, or where multiple a product of the landslide. microfractures occur, confinement is more extensive, and more pressure can be induced. In the Prince Edward Island sedimentary successions, however, fracture surfaces tend to Coastal slope failures in the Gulf of St. Lawrence and be planar and linear, and these (particularly along bedding Northumberland Strait planes) are relatively open, reducing the effectiveness of frost wedging. This factor is to some degree counterbalanced Along the Gulf of St. Lawrence and Northumberland by the friability of the relatively weakly consolidated Strait coastlines, the most common form of slope failure Carboniferous and Permian sediment and by the multitude involves erosion of late Carboniferous and Permian redbeds of small vertical joints likely produced during glacioisostatic of the Bathurst Formation (Davies 1977) and Prince Edward rebound. Island Group (van de Poll 1989). Overlying Quaternary In the stable tectonic environment of Prince Edward deposits are commonly involved, especially on Prince Island, sea-cliff morphology reflects the lithology, fracture Edward Island but, with the exception of frost creep and pattern, glacioisostatic history, and terrestrial erosional small surface debris flows, are not instrumental in failure. processes (notably frost action and glaciation). Marine Incremental slope failures result from frost action coupled processes play a lesser role, as indicated by the similarity of

jointing patterns and rates of coastal cliff development in underlain by Leda clay in Ottawa-St. Lawrence lowlands Prince Edward Island to those developed inland in Atlantic (Cruden et al. 1989). However, even a relatively minor slope Canada (Broster and Burke 1990; Park and Broster 1996; stability problem along a busy transportation route can Catto and St. Croix 1997; White 2002). be costly. The cost of remediation of a failing slope on the Trans-Canada Highway at Gambo, eastern Newfoundland, was estimated at over $650 000 in 1993 (Batterson et al. DISCUSSION 2006). The cost of protective fencing at Upper Island Cove following the 1999 rockfall event was estimated at about The emphasis in this review on Newfoundland and $1 million. Disruption of transportation routes can also Labrador examples is due in part to the extensive archival have an economic effect, as documented by Jackson (2002). research performed in that province compared to others in Temporary closure of the Trans-Canada Highway or other the region. However, other reasons include its greater area, major transportation routes in Atlantic Canada is rare, but greater relief, and higher number of recorded fatalities. In has a major effect on the economy of the area. addition, communities in Newfoundland and Labrador The environmental effects of slope failures can also were mostly founded in locations chosen for access to be considerable. In both Newfoundland and Nova Scotia, good fishing and mining opportunities, whereas the other coarse-sediment flows have resulted in the modification provinces had more diverse economies and many people of both floral and faunal habitats through the removal of settled inland in agricultural communities where a greater vegetative cover and an increase in sediment influx into choice exists for dwelling locations; consequently, fewer the river systems. The catastrophic input of sediments into people live in vulnerable locations. local drainage networks can adversely affect both the water Although a variety of types of slope failures have been chemistry and the submarine ecosystem: in particular, the documented, the majority of incidents that have caused loss life cycle and spawning grounds of Atlantic salmon (Finck of property, loss of life, or disruption to communication/ 1993; Schuster and Highland 2001) transportation corridors relates to debris torrents or flows Where fatalities occur and/or people are displaced from and rockfall, usually block-topple involving single blocks. their homes, as at Harbour Breton, the social impact can be The most common response to landslide problems has devastating. been the implementation of remedial measures, including the installation of protective fencing, gabions, and other elements. Landslide hazard generally has not been CONCLUSIONS incorporated into planning. This is partly because of lack of awareness of landslide hazard in communities, although Slope failures are common in areas such as slope stability is an important consideration in highway Newfoundland and Cape Breton Island; these areas are design. characterized by steep slopes, emergent coastlines and active The success of archival research in Newfoundland, coastal erosion. Mass movements also occur along steep combined with the number of occurrences discovered slopes in remote areas, as in the interior of New Brunswick, by preliminary archival searches in Nova Scotia, strongly but go unreported due to the lack of habitation. The impact suggests that a more systematic examination of newspaper of landslides can vary, from none in remote areas to severe records might yield a fuller record of landslides in Nova where communities and their residents are directly affected Scotia and New Brunswick. through relocation, loss of property or loss of life. Triggering of slope failures can be caused by several In general, landslides in Atlantic Canada are difficult factors, but climatic variables are important, particularly to mitigate. The best strategy seems to be the relocation of for debris flows where heavy rainfall usually precedes residents; but the need to balance cost of such relocations slope failure. Rockfalls can also be triggered by exceptional against any proposed mitigation is recognized. The precipitation, but also are affected by the freeze-thaw identification of potential landslide hazard is important from cycles. Many block-topples occur in spring, after a winter a municipal planning viewpoint. Landslide hazard mapping of repeated freeze-thaw cycles that have widened joints would be an important first step in the recognition of this and fissures and lessened stability. Climatic change may natural hazard within municipalities. Such information affect landslide frequency in the future. Climate modeling should lead to restrictions to development in hazardous suggests an increasing probability of extreme rainfall events areas. Unfortunately even when similar mapping for flooding in Atlantic Canada over the next 50 years, and landslide hazard has been available (e.g., for Badger, Newfoundland frequency may increase as a result. or Fredericton, New Brunswick), development in vulnerable The overall economic effect of landslides in the Atlantic areas has continued unabated. Provinces is minor compared to those of the mountainous areas of British Columbia or densely populated areas

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